Reflect cordon & drain operations on update (#959)

When updating a node, skuba will cordon/drain the node before starting the
operation, and it will uncordon/undrain it whenever the update has been applied
successfully. This might not be expected by the user, so we better document it.

See #774
See SUSE/avant-garde#1842
See SUSE/skuba#1298

Signed-off-by: Miquel Sabaté Solà <msabate@suse.com>

adoc/admin-updates.adoc

@@ -259,6 +259,7 @@ The upgrade via `skuba node upgrade apply` will:
 
 * upgrade the containerized control plane.
 * upgrade the rest of the {kube} system stack (`kubelet`, `cri-o`).
+* temporarily drain/cordon the node before starting the whole process, and then undrain/uncordon the node after the upgrade has been successfully applied.
 * restart services.
 ====